With development of optical fiber communication technologies, in a communications link, a distributed Raman (Raman) fiber amplifier gradually replaces a conventional discrete erbium-doped fiber amplifier by virtue of such features as small crosstalk, low noise, wide spectrum ranges, and high gains. The distributed Raman fiber amplifier injects high-power pump light into an optical fiber in a communications link, and amplifies an optical signal in the optical fiber by utilizing a stimulated Raman scattering effect, thereby implementing long-distance transmission of the optical signal. Power of pump light injected by the distributed Raman amplifier is excessively high, if a fiber cut occurs, pump light leaking out from a fiber-cut location easily causes injury to the human body. Therefore, it is required to detect the fiber cut quickly, and turn off a pump laser of the amplifier or decrease power of the pump light to a safe level in a timely manner, so as to ensure safe operating of a communications system.
In a communications link using a conventional discrete erbium-doped fiber amplifier, output power of signal light transmitted over an optical fiber needs to be detected only at an input end of the amplifier, and when the output power of the signal light is less than a preset threshold, it indicates that signal light leakage occurs during transmission, and then it can be determined that a fiber cut occurs on the optical fiber.
However, in a communications link using a backward pump distributed Raman amplifier (a direction of pump light is opposite to a direction of signal light), the distributed Raman amplifier injects the pump light backwards into an upstream communications optical fiber, and amplifies signal light in the upstream communications optical fiber by utilizing the Raman scattering effect, where the communications optical fiber itself is used as a gain medium. When a fiber cut occurs on the upstream communications optical fiber, the signal light leaks out. However, the pump light in the communications optical fiber generates spontaneous emission light in the fiber, where a waveband of the spontaneous emission light includes a signal optical waveband. The spontaneous emission light is amplified continuously, which results in that a high-power optical signal at the signal optical waveband may still be detected at an input end of the amplifier (an output end of the upstream communications optical fiber). Therefore, a method for detecting signal optical power at an input end or an output end of an amplifier is not suitable for fiber-cut detection.